Numerical simulation of nonlinear optical gain modulation in a Raman fiber amplifier

Nonlinear optical gain modulation (NOGM) in a Raman fiber amplifier is numerically simulated with the generalized nonlinear Schrödinger equation. In the NOGM setup, a single frequency continuous wave seed laser is gain modulated into femtosecond pulses by an ultrafast pump, which can induce strong stimulated Raman scattering in a piece of single mode optical fiber. Different parameters regarding seed, pump and nonlinear gain medium (Raman fiber) are investigated in detail to find the best condition for higher Raman conversion efficiency. Simulated results reveal that the walk-off between pump and Raman pulses due to dispersion is one of the most important factors affecting the NOGM pulse's performance. Only when the speed of walk-off matches with the one of Raman conversion process can the conversion efficiency be optimized. This work offers a guild-line for the design of a fiber-based NOGM laser, which is able to produce wavelength-agile, femtosecond laser pulses with µJ-level pulse energy under more than 85% efficiency.